Fellgiebel was promoted lieutenant colonel in 1933, and became a full colonel (Oberst) the following year. By 1938, he was a major general. That year, he was appointed Chief of the Army's Signal Establishment and Chief of the Wehrmacht's communications liaison to the Supreme Command (OKW). Fellgiebel became General der Nachrichtentruppe (General of the Communications Troops) on 1 August 1940.

In 1942, Fellgiebel was promoted to Chief Signal Officer of Army High Command and of Supreme Command of Armed Forces (German: Chef des Heeresnachrichtenwesens), a position he held until 1944 when he was arrested.[1]

Adolf Hitler did not fully trust Fellgiebel; Hitler considered him too independent-minded, but Hitler needed Fellgiebel's expertise. Fellgiebel was one of the first to understand that the German military should adopt and use the Enigmaencryption machine. As head of Hitler's signal services, Fellgiebel knew every military secret, including Wernher von Braun's rocketry work at the Peenemünde Army Research Center.

Through his acquaintance with Colonel General Ludwig Beck, his superior, and then Beck's successor, Colonel-General Franz Halder, Fellgiebel contacted the anti-Nazi resistance group in the Wehrmacht armed forces. In the 1938 September Conspiracy on the eve of the Munich Agreement, he was supposed to cut communications throughout Germany while Field Marshal Erwin von Witzleben would occupy Berlin.

Fellgiebel was again involved in the preparations for Operation Valkyrie and during the attempt on the Führer's life on 20 July 1944[2] tried to cut Hitler's headquarters at Wolf's Lair in East Prussia off from all telecommunication connections. He only partly succeeded, as he could not prevent the informing of Joseph Goebbels in Berlin via separate SS links. When it became clear that the attempt had failed, Fellgiebel had to override the communications black-out he had set up.

Fellgiebel's most famous act that day was his telephone report to his co-conspirator General Fritz Thiele at the Bendlerblock, after he was informed that Hitler was still alive: "Etwas schreckliches ist passiert! Der Führer lebt!" ("Something awful has happened! The Führer lives!").

Fellgiebel was arrested immediately at Wolf's Lair. He was charged before the Volksgerichtshof ("People's Court"). On 10 August 1944, he was found guilty by Roland Freisler and sentenced to death. He was executed on 4 September 1944 at Plötzensee Prison in Berlin.